The invention relates to the collection of ionic contaminants in media and, in particular, from soils of existing, suspected, or former hazardous waste sites. Specifically, the present invention utilizes the combined advantages of in situ probes, ion permeable components, and electrokinetic phenomena to discretely sample, collect and/or remove unwanted charged species from such media.
For the purposes of the present method and apparatus, the following expression(s) and word(s), unless otherwise indicated, will be understood as having the meaning(s) ascribed thereto by those skilled in the art and/or as otherwise indicated in respect thereto:
1. The word “medium” and the like (whether singular, plural or as a noun or adjective) includes but is not limited to soil, groundwater and/or surface water.
2. The word “soil” and the like (whether as a noun or adjective) includes but is not limited to unconsolidated matter (whether anthropogenic, natural, organic, or inorganic) such as sediment, sand, clay, slurry, mine tailings, and organic material;
3. The word “contaminated” and the like when used in relation to the word “soil” and the like shall be understood as referring to that portion of unconsolidated matter that is contaminated, in any way whatsoever, whether voluntarily or otherwise, and shall be understood as referring to that portion of a “medium” that is contaminated, in any way whatsoever, either voluntary or otherwise;
4. The expression “electrokinetic phenomena” and the like shall be understood as referring to the primary electrokinetic transport mechanisms of electrophoresis, electromigration, and electroosmosis that occur when a direct voltage is applied across electrodes placed in a soil mass. Electrophoresis is the movement of charged colloids (i.e., clay particles, micelles, organic particles, metal oxides) within the pore solution. Electromigration is the movement of ions to their respective electrodes. Electroosmosis (the transport of solutes in the pores of the soil due to the movement of the soil solution in an applied electric field) is particularly relevant here and bears some discussion. Double layer theory in soils postulates (a) the free water layer and (b) the boundary film of water within the diffuse electric double layer surrounding clay particles, organic colloids, and metal oxides. To maintain electroneutrality, counterions are associated with the charged particle surfaces. Under normal hydraulic flow, it is usually assumed that the boundary water film is not affected and only water within the free layer moves under the hydraulic gradient. (In actuality, the two water “layers” are a continuum, and the boundary between the layers is not clearly defined). Under the influence of an electric field, the counterions and their associated water molecules on the particle surfaces will migrate towards their respective electrodes. This movement imparts a net strain on the pore fluid around the hydration shell of the counterions, causing a net shear force to develop through the viscosity of the pore fluid. The net shear force and momentum cause the boundary film of water and free water to move. The thicker the diffuse double layer and the smaller the pore size, the more uniform is the strain field and the farther it extends into the center of the capillary. Because of the pore size effect, electroosmosis is only significant in low permeable soils. Also, since most soil surfaces have net negative charges, the movement of the pore solution due to electroosmosis is usually towards the cathode.
5. The words “hydrogen ion” and “proton” and the like (whether singular, plural or as a noun or adjective) include but are not limited to hydrated proton(s) and hydronium ion(s).
6. The expression “accelerated water splitting” and the like shall be understood as referring to the accelerated dissociation of water into hydrogen and hydroxide ions which occurs at bipolar interfaces between ion exchange membranes and low permeable soils. (See, Desharnais, B. M. and B. G. Lewis “Electrochemical Water Splitting at Bipolar Interfaces of Ion Exchange Membranes and Soils”, Soil Science Society of America Journal, Vol. 66, no. 55 (2002); 1518-1525.) If an ion exchange membrane placed in contact with a soil has an electrostatic charge opposite in sign to the predominant charge on the soil colloidal particles, the interface is, in effect, bipolar. If an external electric field is then applied across the interface, conditions can give rise to accelerated water splitting, similar to the conditions found in bipolar ion exchange membranes. Accelerated water splitting occurs when the free pore solution in the low permeable soil moves away from the bipolar interfaces due to electroosmosis, thus causing an unsaturated zone at these interfaces. Accelerated water splitting then initiates at these interfaces since there are not enough counterions in contact with the IEMs to maintain an ionic current.
Sampling, collection, characterization, monitoring and/or removal are necessary and integral components of the cleanup of contaminated soils, aquifers, groundwaters and the like. Sampling and monitoring are used to evaluate the extent of contamination before remediation and the soil-cleanup efficiencies after remediation. The accurateness of this monitoring will dictate not only the type of remediation to be implemented but also the cleanup expense of the site. The prior art includes several techniques, the disadvantages of which have prompted the search for an improved apparatus and methodology, particularly for sampling and/or collecting low concentrations of cationic and anionic contaminants. Examples of such cationic contaminants include heavy metals over the range of encountered oxidation states (i.e., lead, mercury, cadmium, nickel, copper, zinc, and chromium), radioactive cationic species (i.e., radium, cesium, strontium, cobalt, and uranium), and hazardous organic cations (i.e., organic bases such as aniline and pyridine). Examples of such anionic contaminants include toxic anions (i.e., nitrates, chromates, and selenates) and hazardous organic anions (i.e., organic acids such as chlorophenols, nitrophenols, phenols, and cresols).
The majority of site explorations in the United States involve some form of media invasion, i.e., submersion, drilling or boring. In the context of contaminated soil cleanup, the creation of boreholes for soil sampling and the installation of monitoring wells and lysimeters for solution sampling all involve drilling. Of the many problems associated with drilling, a primary issue is the handling of contaminated cuttings. Direct contact with contaminated soils and solutions should be avoided to insure the health and safety of the workers. In extreme cases, it may be necessary to have full body coverage and separate air supplies. Risks are minimized if cuttings and fluids are minimized and small samples are collected. However, the tradeoff with small samples is the risk of not collecting a detectable or representative amount of the contaminant. Consideration should also be given to the contamination of the sampler and equipment, which may contaminate other samples. Second, drilling large boreholes and placing monitoring equipment such as lysimeters, pumps, and casings may significantly disturb the surrounding soil. A third problem is cost. Drilling and boring costs can range from about $66/m ($20/ft) to $3300/m ($1,000/ft) and higher. A fourth problem is ease of use. Drilling techniques such as hollow stem augers, direct rotary mud drilling, and core drilling may be complicated and time consuming. Pumping considerations, drilling fluid viscosities, and correct selection of rotary speeds and applied axial forces all significantly complicate drilling.
Disadvantages are also associated with media samplers. Again, in the soil context, as with all soil samplers (i.e., those which actually remove soil from the subsurface), the act of soil extraction may chemically and physically disrupt the soil sample. Likewise, some samplers do not collect representative samples. Studies have shown that suction lysimeters may collect samples that have a different chemical composition than the actual pore solution. In addition, the removal of the soil does not lend itself to repeatedly sample the same aquifer location again. Soil sampling is necessary, however, to measure the actual concentration of contaminants as no direct in-situ sampling techniques exist in the prior art. Further, very few samplers are effective in both the saturated and unsaturated zones. Aside from samplers that extract soil, few samplers are effective in low permeability clays and silts. Yet another problem is that some sampling techniques are time consuming. For example, suction lysimeters may require several days to fill.
The aforementioned concerns and deficiencies are discussed in the soil context, but comparable issues persist with other contaminated media, such as ground and surface water. Considering the above, there exists a need for innovative, accurate, and cost-effective monitoring sampling, collection and/or removal techniques.